An internal combustion engine typically includes one or more valves that allow fresh air to enter a combustion chamber of the engine and/or allow exhaust gases to exit from the combustion chamber. These engine valves, particularly the exhaust valves, are subjected to very high temperatures during operation of the engine. For example, the valves may often reach temperatures of 800° C. or higher (e.g. 800 to 850° C.), albeit only for a small fraction of the engine's overall life because of the heat released by combustion of fuel in the combustion chamber. Conventional materials used to make the engine valves can survive such high temperatures for a relatively short period of time, for example, up to 2000 hours, after which the engine valves may require repair or replacement. Performing such maintenance on engine valves, however, requires taking the engine out of service, and further involves time and expenses associated with the required repairs. Therefore, it is desirable to increase the useful life of the engine valves. For example, it may be desirable to increase the useful life of engine valves nearly ten-fold, ranging between 10,000 hours and 30,000 hours.
Engine valves are typically made of wrought alloys, such as nickel-base superalloys, and are typically manufactured using a forging process. Changing the composition of the wrought material to increase its ability to withstand high temperatures usually reduces the ductility of the material, making it harder to use manufacturing processes like forging, rolling, and/or extrusion. Furthermore, the reduced ductility may also cause cracking of the valves during manufacture, significantly reducing yield and increasing manufacturing costs. Therefore, it is desirable to develop an alloy material that can withstand repeated exposure to temperatures of 850° C. or more for over 10,000 to 30,000 hours, and that is suitable for manufacturing valves using a manufacturing process, such as casting.
U.S. Pat. No. 3,164,465 to Thielemann (“the '465 patent”) that issued on Jan. 5, 1965 discloses a nonferrous nickel-based alloy suitable for use with a casting process, and having corrosion resistance and mechanical strength at temperatures up to about 2000° F. (i.e. 1093° C.). The '465 patent discloses a preferred alloy composition that includes the following percentages by weight of various constituent elements: about 8.75% to about 10.25% of chromium, about 11% to about 16% of tungsten, about 0.8% to about 1.8% of columbium and/or tantalum, about 4.75% to about 5.5% of aluminum, about 0.75% to about 2.5% of titanium with the provision that the amount of titanium does not exceed the amount of aluminum, about 8% to about 12% of cobalt, at least one metal in the amounts indicated selected from the elements consisting of about 0.03% to 0.12% of zirconium, about 0.01% to about 0.03% of boron, about 0.12% to about 0.17% of carbon, about 1.5% maximum of iron, about 0.10% maximum of silicon, about 0.1% maximum of manganese, and with the remainder being nickel and incidental impurities, with the nickel content being in the range of about 50% to 77%. The '465 patent discloses that molybdenum is optional but if present may not exceed 3% by weight. The '465 patent also discloses that the constituent composition must preferably satisfy the equation 1 X % Cr+1.1 X % W+3.4 X % Cb or Ta+4.3 X % Ti+6 X % Al=60 to 70. The '465 patent discloses that it is preferred to maintain the zirconium to boron ratio at about 4:1 to maintain the high temperature metallurgical stability and strength characteristics of the disclosed alloy.
Although the alloy disclosed in the '465 patent may provide improved mechanical properties at higher temperatures, still further improvements in the material characteristics may be possible. In particular, the alloy disclosed in the '465 patent may develop micro-pores during the casting process. Micro-pores in the finished valves may produce regions of stress concentration, which in turn may cause early onset of fatigue crack initiation, particularly when repeatedly exposed to high temperatures. Additionally, the disclosed alloy composition of the '465 patent may be susceptible to precipitation of M6C carbides at high temperatures. M6C carbides have a plate morphology and can reduce the high temperature strength and reduce ductility of the material disclosed in the '465 patent
The high temperature alloy of the present disclosure solves one or more of the problems set forth above and/or other problems in the art.